Municipal wastes are disposed of by dumping for landfill or by incineration. Since the available landfill space is limited for the growing volume of municipal refuse, most of the latter is of necessity incinerated.
Separating wastes which cannot burn or should not be burned down from burnables prior to incineration is ideal. However, perfect sorting at the time of waste collection is generally impracticable. Consequently, urban wastes frequently contain heavy metals. For example, such heavy metals as cadmium, lead, zinc, copper, manganese, chromium, and mercury are found in the gaseous emissions from municipal incineration plants.
Municipal refuse in recent years has had increasing proportions of mercury-bearing wastes such as dry cells and fluorescent tubes, and it is not infrequent that mercury is detected in municipal incinerator emissions. Unlike other heavy metals, mercury in the emissions does not stick to soot and other particulates. It is therefore not recovered by an electric dust precipitator or the like but is released in the form of mercury vapor into the atmosphere.
Various proposals have hitherto been made for the removal of mercury vapor, or separation of mercury vapor from the gaseous phase. Roughly, they are classified into (1) the use of a strong oxidizing agent, (2) adoption of a solid mercury adsorbent, and (3) a thiourea-copper salt process.
The first approach, the use of a strong oxidizing agent, comprises collecting mercury vapor by means of a mixed solution, e.g., of potassium permanganate and sulfuric acid. The process is utilized in mercury removal from hydrogen gas that results from the electrolysis of salt in soda-manufacturing plants. Since the process depends on the strong oxidizing power of the mixed potassium permanganatesulfuric acid solution for the oxidative absorption of mercury vapor, it presents a number of problems for the maintenance of the equipment materials and also concerning the disposal of the waste liquid that has absorbed mercury. These problems render the adoption of the process extremely difficult or practically impossible for municipal incinerators that emit very large volumes of gases.
For the treatment of mercury in mercury-containing waste liquids, coagulating sedimentation with sodium sulfide is in extensive use. If sodium sulfide for this purpose is added to excess, mercury sulfide once formed could be redissolved in the waste liquid. Also, the addition of an acid for the pH adjustment of the waste liquid would acidify the liquid, thus evolving hydrogen sulfide. The hydrogen sulfide gas produced in this way would not only deteriorate the working environments but also, when released to the atmosphere, would lead to air pollution. The deposit of hydrogen sulfide formed by coagulating sedimentation is instable, as noted above, and has compelled the addition of a sophisticated treatment with active charcoal, chelate resin or the like to the stage that follows the coagulating sedimentation. Along with such a sophisticated treatment it is customary to introduce a large percentage of a coagulant or coagulant aids to the process of coagulating sedimentation in order to prevent the redissolution of the settled hydrogen sulfide. The coagulant or aids thus added eventually increase the quantity of sludge and decrease the mercury recovery efficiency.
The second process using a solid mercury absorbent consists in removing mercury vapor by adsorption from a gaseous phase by the use of an active-charcoal-based solid mercury adsorbent, chelate resin or the like. The process, in use at soda plants, is limited in adsorption capacity and is ordinarily intended for the removal of atomic mercury. It cannot be said effective as a process for mercury removal from municipal incinerator emissions wherein atomic mercury accounts for only 10 to 40% of the mercury content.
In order to achieve the end of mercury removal from gaseous emissions not realized by the first and second approaches, the third process was proposed (Japanese patent application Publication No. 4291/1981). The process for mercury removal is characterized by bringing a gaseous mixture containing mercury vapor into contact with an aqueous solution of thiourea and a copper salt at 50.degree. C. or above to allow the mercury vapor in the gaseous phase to be absorbed for removal by the liquid phase.
The third proposal is not considered desirable, either, since it employs thiourea, a highly toxic and cacinogenic substance according to reports on animal experiments. Moreover, if mercury vapor is to be removed by adsorption from municipal incinerator emissions, a considerably large quantity of thiourea, e.g., more than 5,000 mg/l, will have to be used. Usually, such a large quantity of thiourea would boost the chemical oxygen demand (COD) of the waste liquid. When 5,000 mg of thiourea was added per liter in conformity to the procedure described in the specification of the cited invention, the COD was found to rise to as much as 3,000 to 3,500 mg/l. Waste liquid with COD at such a high level cannot be directly drained off; some extraordinary equipment for oxidative decomposition of the waste liquid is indispensable. Further, an increase in the rate of thigurea addition necessitates a corresponding increase in the amount of the coagulant to be used. It will eventually lead to increased sludge formation and reduced mercury recovery efficiency.
Accordingly it is a principal object of the present invention to provide a technique of mercury removal at very high efficiency from municipal incinerator emissions.
It is another object of the invention to provide a process for cleaning municipal incinerator emissions which is adequately applicable to existing refuse incinerators and which permits removal of mercury vapor without the necessity of special facilities therefor.
Another object of the invention is to provide a process for cleaning municipal incinerator emissions which does not require any extraordinary process step or facility but can utilize ordinary equipment at low maintenance costs.
Still another object of the invention is to provide a process for cleaning municipal incinerator emissions whereby mercury is precipitated in a stable form, the coagulant requirement is decreased, and accordingly the quantity of resulting sludge is reduced to heighten the mercury concentration in the sludge and thereby enchance the efficiency of mercury recovery.
Yet another object of the invention is to provide a process for cleaning municipal incinerator emissions which makes use of conventional equipment for waste gas scrubbing and waste liquid treatment and can remove mercury as well as hydrogen chloride and sulfur oxides from waste gases by absorption into a liquid phase.
A further object of the invention is to provide a process for waste gas cleaning which is satisfactorily applicable to the removal of mercury from not only the emissions out of municipal refuse incinerators but also from other gases containing mercury vapor, such as hydrogen gas generated by the aforementioned electrolysis plants.
The present inventors have made extensive experiments and investigations in an effort to overcome the difficulties of the prior art and achieve the objects enumerated above. As a result, it has now been found that a large quantity of mercury can be most efficiently removed by making effective use of the physical properties of mercury (Hg) present in the gaseous phase; that is, the mercury in the gaseous phase can be converted by an acidic gas into an easily water-soluble mercuric chloride (HgCl.sup.2) which then can be fixed by a saline solution or the like to be a chlorocomplex ion (HgCl.sub.4.sup.-2) stable in liquid, the process itself being readily carried into practice through full utilization of existing equipment. The present invention is predicated upon these findings.